1. Field of the Invention
This invention generally relates to the field of electrical engineering and, more particularly, to an arrangement for protecting power transistors from being damaged by inadvertant improper operation of circuitry controlling the transistors.
2. Description of the Prior Art
It is known that if conductors subject to power surges are passed through beads of magnetic material, the beads are capable of absorbing short pulses of energy thereby protecting electrical components connected to the conductors. This principle is taught, for example, in U.S. Pat. No. 2,594,890 to Ellwood issued Apr. 29, 1952. Ellwood discloses ferrite beads strung on a wire for the purpose of protecting electrical contacts from deleterious erosion caused by electrical arcing and/or energy dissipation during contact operation. In the Ellwood circuit, a reed switch is connected in series and controls a relay coil. When the reed switch is closed, a d.c. current flows in the circuit, and when the reed switch is opened, the relay coil inductance produces whatever voltage is necessary to maintain the current flow. This results in an arc being created across the reed switch contacts. In order to protect the reed switch contacts from arcing, Ellwood places three ferrite beads on the conductor between the reed switch and the relay coil. However, the protection afforded by these beads decreases if the material of the beads approaches saturation. In the Ellwood circuit, the d.c. current which flows when the reed switch is closed biases the beads towards saturation, and this is apparently the reason that Ellwood uses three beads to provide the requisite protection for the reed switch contacts.
Also known in the prior art are reset circuits for saturable reactors. U.S. Pat. No. 4,275,430 to Seki et al. issued June 23, 1981, discloses a snubber circuit connected between the anode and cathode of a gate turn-off (GTO) thyristor. A saturable reactor is connected in series to the GTO thyristor. A gate off signal is supplied to the gate of the GTO thyristor and to the saturable reactor as a backward current to reset the saturable reactor.
U.S. Pat. No. 4,323,840 to Gawler issued Apr. 6, 1982, discloses a switching mode regulator which is improved in transient performance by the addition of a current transformer which aids in storing energy from the turn-on transient. The current transformer prevents large currents due to finite recovery time of a flyback diode. By using a capacitor with the transformer, energy is returned to the load in a manner which reduces the time rate of change of voltage applied to the switch at turn-off.